ECN - July 2015

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integration to achieve optimal results. Typically, for adhesion and mechanical and thermal robustness, three-layer metallization schemes are used. A common scheme is titanium/platinum/gold (Ti/Pt/Au), often used in high-end devices because of its excellent stability and endurance characteristics. However, because the thermal conductivities of titanium and platinum are relatively low, researchers sought, and found, an improved material: chromium (Cr).

Chromium forms a carbide with diamond and is also readily used as a barrier layer, enabling it to perform both functions at a relatively high thermal conductivity of Tc=93.9 W/mk. To test the thermal effectiveness of chromium, samples were prepared at the Centre for Device Thermography and Reliability at Bristol University comparing a standard Ti/Pt/ Au (100/120/500nm) metallization with this novel Cr/Au (100/500nm) configuration. Results showed the effective thermal conductivity of the Cr/Au metallization to be three to four times higher, compared to Ti/Pt/Au.

To demonstrate the advantages of a Cr/Au metallization compared to Ti/Pt/Au, high power GaN-on-Silicon Carbide (SiC) high electron mobility transistor (HEMT) devices were mounted to a CVD diamond heat spreader. To ensure comparable results, all samples were placed on a temperature stable platform also made from high thermally conductive diamond material. Results are shown in Figure 3. In the left diagram, the base temperature is plotted for increasing power output from the device. The temperature for the Cr/Au configuration is significantly lower, at 9 W device power output by about 10 °C. On the right hand side, the graph shows the temperature as measured on the transistor channel directly. In this case, the lower thermal resistivity of the Cr/Au metallization decreases the channel temperature by more than 20 degrees C at 9 W power output. The significantly lower temperature will result in as much as a four times longer device lifetime, or alternatively, will allow such devices to be packaged smaller and with higher power densities, according to a University of Bristol paper titled “ GaN-on-Diamond High-Electron-Mobility Transistor – Impact of Contact and Transition Layers”.

GaN-on-diamond wafers: The next step in improved
device architecture

It’s clear, from extensive research and positive results leveraging synthetic diamond heat spreaders, that there is room to continue modifying device architecture for improved performance. Engineers have long known the potential of GaN to help create a new generation of smaller and faster devices with greater power density.